Nanoelectronic devices are generally fabricated on semiconductor substrates as integrated circuits. A complementary metal-oxide-semiconductor (CMOS) field-effect transistor is one of the core elements of the integrated circuits. Dimensions and operating voltages of CMOS transistors are continuously reduced, or scaled down, to obtain ever-higher performance and packaging density of the integrated circuits.
One of the problems due to the scaling down of CMOS transistors is that the power consumption keeps increasing. This is partly because leakage currents are increasing (e.g., due to short channel effects) and because it becomes difficult to decrease the supply voltage. The latter is mainly due to the fact that the subthreshold swing is limited to minimally about 60 mV/decade, such that switching the transistor from ON to OFF needs a certain voltage variation and therefore a minimum supply voltage.
Tunnel field-effect transistors (TFETs) are typically advertised as successors of metal-oxide semiconductor field-effect transistors (MOSFETs), because of their absence of short-channel effects and because of their resulting low off-currents. Another advantage of TFETs is that the subthreshold swing can be less than 60 mV/dec, the physical limit of conventional MOSFETs, such that potentially lower supply voltages can be used. However, all-silicon TFETs typically suffer from low on-currents, a drawback related to the large resistance of the tunnel barrier.
In the past there have been different attempts to improve TFET performance.
For example, Verhulst et al. propose in Electron Dev. Lett. 29, 1398 (2008) to use a heterostructure to boost the on-current. The heterostructure consists of using one semiconductor material in the source and another semiconductor material in the channel, such that the effective energetic distance between the source valence band and the channel conduction band (for n-TFET) has become smaller than the bandgap of the individual materials. This smaller effective bandgap allows for more efficient tunneling and hence higher on-currents.